At the moment, in esxi 5.0, memory compression occurs when the page's compression ration is greather than 50% (so says the "Understanding Memory Resource Management in VMware vSphere 5.0" guide). This is not tunable, but it might be nice to be able to given that it will be much faster than hypervisor swapping.
Does anyone think this might be a sensible parameter to ask VMware for, or would it just be too "dangerous"?